Maybe but the bottom line is that facet of his team consistently underperformed. Now DD did not completely ignore it, acquiring relievers in the off-season. (Of course, at one point his "answer" was 39yo Joe Nathan. Sure he did get a reliever, but let;s get real. Nathan was far from his best days.) But he also did not make changes when those same flaws kept arising.
It's funny - here in Boston fans complained in the post-Nomar years that the team just could not find a SS they liked. As the team went through Cabrera and Renteria and Gonzalez and Lugo and Scutaro, etc. And that team also made the post-season every year but one.
If you want definitive proof that his weak bullpen was not the factor in winning a title, fine. But are you going to stand by the "good enough" argument to justify it being ignored year after year while the window was open for the Tigers as their team came up just short despite having three surefire Hall of Famers on the roster?